A phase retarder is a film or sheet showing birefringence. Since a phase retarder has different refractive indices in two directions crossing at right angles to each other, light transmittance by the phase retarder produces light rays whose planes of polarization make a phase difference.
Phase retarders which are currently available on the market and have been in practical use include a so-called .lambda./4 retarder or quarter retarder having a function to cause incident light having a wavelength of .lambda. to produce a phase difference of .lambda./4. The quarter retarder is prepared by uniaxially stretching a cellulose acetate-based film. The quarter retarder serves as a circular polarizer when it is combined with a linear polarizer in such a manner that the optical axis thereof makes an angle of 45.degree. with respect to the planes of polarization of the linear polarizer. A linear polarizer is also utilized as anti-glare materials, for example, a VDT (visual display terminal) filter, making use of its function to cut reflected light.
In addition to the above-described cellulose type resins, high polymeric materials which have been recommended to constitute the .lambda./4 retarder include polyvinyl chloride resins as disclosed in Japanese Patent Publication No. 34477/70 and Japanese Patent Application (OPI) No. 125702/81 (the term "OPI" as used herein means an "unexamined published Japanese patent application"); polycarbonate resins as disclosed in Japanese Patent Publication No. 12190/66 and Japanese Patent Application (OPI) No. 130703/81; polyacrylonitrile resins as disclosed in Japanese Patent Application (OPI) No. 130702/81; polystyrene resins as disclosed in Japanese Patent Application (OPI) No. 125703/81; polyolefin resins as disclosed in Japanese Patent Application (OPI) No. 24502/85; and the like. However, all the conventional phase retarders made of these materials are .lambda./4 retarders whose retardation value (hereinafter referred to as R value) is in the vicinity of 135 nm. None of the above-cited publications refers to a process for producing a .lambda./2 retarder or a full retarder. The terminology "R value" as used herein means a product of a thickness (t) of a film or sheet and a birefringence (.DELTA.n) of the film or sheet and can be represented by equation: EQU R=.DELTA.n.times.t
Recent studies have been directed to application of a .lambda./2 retarder or a full retarder which causes a phase difference of .lambda./2 or .lambda., respectively. Under the present situation, however, there has not yet been proposed any phase retarder having satisfactory quality, that is, having an R value between 200 and 625 nm, preferably between 200 and 350 nm (.lambda./2 retarder) or between 475 and 625 nm (full retarder) and being free from optical color unevenness.
In liquid crystal display systems, it has been proposed to improve image quality of TN (twisted nematic) type liquid crystal displays, in which nematic liquid crystal molecules have a twisted angle of 90.degree. and a liquid crystal cell is sandwiched by a pair of polarizing sheets with their planes of polarization crossing or being parallel, by interposing a phase retarder between one of the polarizing sheets and the liquid crystal cell, as disclosed in Japanese Patent Application (OPI) Nos. 186937/86 and 26322/85.
To cope with the demands for an increased display capacity and an enlarged display screen, generally called STN (super twisted nematic) type liquid crystal displays have recently been developed, in which the nematic liquid crystal molecules are twisted at an angle of about 180.degree. to 270.degree.. However, the STN type liquid crystal display suffers from coloring ascribed to birefringence of liquid crystal molecules, resulting in the failure of B/W display. In order to solve this problem, it has been suggested that a liquid crystal cell for color removal is added to the STN type liquid crystal cell as an optical compensator to thereby eliminate the color and to enable B/W display, as described, e.g., in the October issue of Nikkei Micro Device, 84 (1987). It would be possible, in principle, to use a phase retarder in place of the above-described liquid crystal cell for color removal.
Conventional quarter retarders, however, do not serve for the new uses including these liquid crystal display systems for reasons that 1) the retardation value (hereinafter defined) does not meet with the purposes, 2) the optical axis thereof is not fixed, 3) they suffer considerable optical color unevenness, and the like. That is, it has been keenly demanded to develop a phase retarder having a fixed optical axis, freedom from optical color unevenness and a retardation value between 200 nm and 1000 nm, which would be promising for application to the above-described new optical uses including liquid crystal displays, but none of the state-of-the-art phase retarders satisfies these requirements.